Digital Video Broadcasting (DVB) systems are well-established. Content streams, such as video and audio broadcasts, are broadcast in digital form using coding and modulation schemes which allow efficient use to be made of the available transmission spectrum. DVB content can be delivered by terrestrial transmission networks or by satellite transmission networks. In either case, there is a need to transport a plurality of DVB content streams over a transmission channel which has a finite bandwidth. FIG. 1 schematically shows a DVB transmission system. A plurality of content streams 10, such as audio and video data, may originate from a variety of different suppliers. The streams are coded, multiplexed and modulated onto a delivery bearer by a transmission formatting function 30. In the case of the satellite distribution network shown in FIG. 1, the delivery bearer is an uplink bearer 61 between a ground station 60 and a communications satellite 65. The satellite 65 translates the delivery bearer to a different part of the frequency spectrum before broadcasting the content over a region 43 to receiving terminals 75 at user premises 70.
FIG. 2 shows an example transmission formatting function which would be used in the transmission chain of a Digital Video Broadcasting over Satellite 1 (DVB-S1) system. The main parts of the formatting function 30 are an encoding/multiplexing stage 40 and a modulating stage 50. The encoding stage 40 has a plurality of inputs 10, each representing a stream of audio/video data. Each stream is encoded by an encoder 41 using coding schemes such as MPEG-2. Such coding schemes generate a variable rate data stream, with the bit rate varying according to the complexity of the input stream. A video stream with a fast-moving or finely detailed image will cause the encoder to generate a higher data rate than a video stream with a static image or an image without fine detail. Similar schemes are used for coding audio data. The sum of the group of N coded outputs will vary according to the complexity of the individual input signals, at any time. A multiplexer 45 combines the coded data streams onto a link 48. The modulation stage 50 modulates the coded data onto a bearer having a finite capacity of B Mbaud. The modulation stage 50 uses a single modulation scheme and thus the relationship between the output rate of the multiplexer 45 and the modulation stage 50 is a straightforward relationship. As noted above, the transmission bearer 58 has a finite bandwidth which must be shared by the plurality of content streams. Coding units 41 can be adjusted, on a real-time basis, to apply a more harsh or lenient coding strategy to the input signals 1 such that the combined output of the multiplexer 45 is at, or less than, the maximum possible rate.
There is an ongoing desire to use the limited radio spectrum for satellite services in a more efficient manner. Digital Video Broadcasting over Satellite 2 (DVB-S2) aims to provide an increased performance over DVB-S1 by a choice of modulation methods, tighter channel roll-off and a more powerful forward error correction (FEC) scheme. Referring again to FIG. 2, the modulation stage 50 in a DVB-S2 system can use a range of possible modulation schemes, applying different modulation schemes to different streams and even changing a modulation scheme for any given stream. A consequence of the increased complexity of DVB-S2 is that the bit rate output by the multiplexer will no longer be related to the modulation stage and the capacity of the bearer 58 in a straight forward manner. This makes it considerably more difficult to multiplex content streams onto a bearer in real time. If the coding/multiplexing stage 40 generates too much data then buffers at the modulation stage 50 will begin to fill or overflow, incurring a transmission delay in the individual data streams or even loss of data. Conversely, if coder/multiplexer 40 applies a coding strategy which is unduly harsh, this will degrade the quality of the individual video streams.
Accordingly, the present invention seeks to provide an improved transmission formatting arrangement.